Active Power Filter

ABSTRACT

An active power filter comprises an energy storage capacitor, an inverter, a filtering circuit and a controller. The inverter is controlled to act as a virtual resister at a fundamental frequency for compensating for the power loss of the active power filter, act as a virtual capacitor at a fundamental frequency for compensating for a fundamental reactive power of the load, and/or generate a harmonic current for suppressing the harmonic currents of specific orders of the load.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The presented invention relates to an active power filter. Moreparticularly, the presented invention relates to a parallel connectionof an active power filter and a load for providing a selectablecompensation from a fundamental reactive current, an attenuation of acompound of the harmonic currents of specific orders, and a combinationof both of them, with the load being particularly a nonlinear load andan attenuating ratio of the attenuation being adjustable.

2. Description of the Related Art

Recently, the characteristics of power electronic devices have beenimproved significantly. The power electronic devices withcharacteristics of high-voltage rating, high-current rating, and highswitching speed have been developed due to the improvement ofsemiconductor manufacturing technique. Power electronic devices arewidely applied in electric power equipment, such as uninterruptiblepower supply, motor driver, arc furnace, trolley car, battery charger,and lighting appliance etc. The electric power equipment may generate alarge amount of harmonic currents due to the nonlinear inputcharacteristic of such loads. The harmonic current will pollute thepower system and result in the serious problems such as transformeroverheat, rotary machine vibration, degrading voltage quality, electricpower components destruction, medical facilities malfunction etc. Inorder to improve the problems of harmonic pollution effectively, manyharmonic control standards, such as IEEE519-1992, IEC1000-3-5, andIEC1000-3-4 etc., have been established by international researchcenters.

Traditionally, passive power filter configured by the inductor andcapacitor was used to solve the problems caused by the harmonic in apower system. However, the passive power filter may cause resonance andneighbor harmonic current injection problems that may damage the passivepower filter. Additionally, it is quite hard to obtain a betterfiltering performance of the passive power filter due to the filtercharacteristic of the passive power filter may be affected by the systemimpedance.

Because of the mentioned drawbacks of the passive power filter, activepower filters are developed for harmonic suppression recently, whichhave a conventional structure as shown in FIG. 1. A power source 91provides a load 92 with electrical power, and an active power filter 93connects with the load 92 in parallel for providing a compensatingcurrent, wherein the compensating current equals to the fundamentalreactive current and the harmonic current of the load. Subsequently, thecompensating current injects into a power line between the power source91 and the load 92 to suppress the harmonic current of the load 92 andimprove the power factor simultaneously.

The conventional active power filters disclosed in U.S. Pat. No.5,677,832 and U.S. Pat. No. 5,614,770 with a control structure as shownin FIG. 2. A calculating circuit 80 detects a load current, a sourcevoltage and a DC voltage of the active power filter for calculating areference signal of compensating current. An output current of theactive power filter is detected and subtracted from the reference signalof compensating current by a subtracter 81, and the subtracting resultis transmitted to a current controller 82 to generate a control signal.Although the conventional active power filter is able to suppress theharmonic current and improve the power factor, there are still somedisadvantages as follows:

-   -   1. a conversion ratio of a current sensor for the load current        and that for the output current of the active power filter        should be pre-known, otherwise, the compensation performance of        the active power filter will be degraded due to the mismatch of        the compensating current of the active power filter and the        harmonic current of the load; and    -   2. a generated compensating current includes the harmonic        currents composed of all orders and the fundamental reactive        current, which results in application limitation in some        applications.

A direct source current compensation method disclosed in “Simplifiedcontrol method for the single phase active power filter,” IEE Proc.Electrical Power Applications, vol. 143, 1996, pp. 219-224 was proposedto solve the first disadvantage above. Nevertheless, its compensatingcurrent still includes the harmonic currents composed of all orders andthe fundamental reactive current.

Although, an active power filter disclosed in U.S. Pat. No. 5,977,660senses the source current to calculate a reference signal of acompensating current thereby, the generated compensating current stillsuppresses the harmonic currents of all orders and compensates thefundamental reactive current simultaneously. In addition,microprocessors are generally used to implement the controllers ofconventional active power filters. Since the fundamental componentdominates the source current after compensating by the active powerfilter, the harmonic components are too small to be detected from thesource current. Especially due to the bits limitations in A/D convertersof microprocessor, an evident error will appear in the calculation ofreference signal of compensating current for the active power filter. Aninaccurate compensating current resulted form the above reasons willdegrade the filtering performance of the active power filter.

The presented invention intends to provide an active power filterwithout those disadvantages of the conventional ones. The control methodof proposed active power filter detects a source current, a sourcevoltage and an energy storage capacitor voltage of the active powerfilter to calculate a reference signal of compensating current. Thereference signal of compensating current can be set manually to selectas a fundamental reactive current, an attenuation of a compound of theharmonic currents of specific orders, and a combination of both of them.Moreover, an attenuating ratio of harmonic currents of each specifiedorders is adjustable individually. For improving the accuracy in thecalculation of reference signal of compensating current, the detectedsource current is separated into two parts before transmitted to amicroprocessor for calculating a reference signal of compensatingcurrent. One is the source current itself, and the other part is thecombination of all harmonic components. Furthermore, the control methodalso operates the active power filter as parallel operation of a virtualresister and a virtual capacitor at the fundamental frequency tocompensate for the fundamental reactive power of the load and the powerloss of the active power filter.

SUMMARY OF THE INVENTION

The primary objective of this invention is to provide an active powerfilter, which utilizes a controller operating an inverter to generate acompensating current injecting into a power line connected between apower source and a load. Consequently, the presented inventioneffectively suppresses the harmonic currents and/or improves the powerfactor.

Further scope of the applicability of the presented invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various willbecome apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The presented invention will become more fully understood from thedetailed description given herein below and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the presented invention, and wherein:

FIG. 1 is a schematic circuitry of a power system with a conventionalactive power filter;

FIG. 2 is a schematic circuitry of a conventional active power filter;

FIG. 3 is a schematic circuitry of a power system with an active powerfilter in accordance with a first embodiment of the presented invention;

FIG. 4 is a schematic circuitry of a controller of the active powerfilter in accordance with a first embodiment of the presented invention;

FIG. 5 is an equivalent circuit of the power system of an nth harmonicorder in accordance with the first embodiment of the presentedinvention;

FIG. 6 is a schematic circuitry of a power system with an active powerfilter in accordance with a second embodiment of the presentedinvention;

FIG. 7 is a schematic circuitry of a power system with an active powerfilter in accordance with a third embodiment of the presented invention;

FIG. 8 is a schematic circuitry of a power system with an active powerfilter in accordance with a fourth embodiment of the presentedinvention;

FIG. 9 is a schematic circuitry of a power system with an active powerfilter in accordance with a fifth embodiment of the presented invention;and

FIG. 10 is a schematic circuitry of a power system with an active powerfilter in accordance with a sixth embodiment of the presented invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3, a schematic circuitry of a power system with anactive power filter in accordance with a first embodiment of thepresented invention is illustrated. The active power filter 3 is appliedin a three-phase four-wire power system that further includes a powersource 1, a load 2, and a power line set 4. The power source 1 is athree-phase four-wire AC power source and supplies the load 2 withelectrical power through the power line set 4 comprising three powerlines 40, 41, 42 for the three phases respectively and a neutral line43. The active power filter 3 electrically connects to the power lineset between the power source 1 and the load 2.

Still referring to FIG. 3, the active power filter 3 comprises an energystorage capacitor 30, an inverter 31, a filtering circuit 32, and acontroller 33. The energy storage capacitor 30 provides a regulated DCvoltage and is consisted of two serially connecting DC capacitors,wherein the connected point of the two DC capacitors connects to theneutral line 43. The inverter 31 electrically connects to two terminalsof the energy storage capacitor 30. The inverter is a three arms bridgestructure with split DC capacitors. The three arm bridge structureconsisted of six power electronic switches. In operation, switching thepower electronic switches of the inverter 31 converts the DC voltage ofthe energy storage capacitor 30 into a compensating voltage. Thefiltering circuit 32 has two sets of terminals with one of themelectrically connects to the inverter 31 and the other connects to thethree power lines 40, 41, 42 between the power source 1 and the load 2.The filtering circuit 32 comprises a plurality of inductors, capacitorsand resisters, and acts as a low pass filter to filter the ripples dueto the high-frequency switching of the inverter 31. Through thefiltering circuit 32, the compensating voltage generated by the inverter31 converts into a compensating current. The compensating currentinjects into the power line set 4 for compensating for the power loss ofthe active power filter and selectively compensating the load 2 for acompound of the harmonic currents of specific orders and/or afundamental reactive current. Consequently, the harmonic current of thesource current is suppressed and/or the power factor is improved.

Referring to FIG. 4, a schematic circuitry of the controller 33 of theactive power filter 3 in accordance with a first embodiment of thepresented invention is illustrated, with the inverter 31 being in acurrent-mode control. The controller 33 comprises a harmonic currentloop 331, a fundamental reactive current loop 332 and a fundamental realcurrent loop 333. A reference signal I_(r)* of the compensating currentis obtained through the three circuits 331, 332, 333, and an outputcurrent of the active power filter 3 is detected. As a result, controlsignals for the inverter 31 are produced by utilizing the controller 33completing a feedback control with the reference signal I_(r)* and theoutput current of the active power filter 3, so that the output currentof the active power filter 3 can track the reference signal I_(r)*.

Still referring to FIG. 4, firstly, the harmonic current loop 331 of thecontroller 33 of the active power filter 3 is used to obtain a harmoniccomponent of the reference signal I_(r)*. The harmonic current loop 331comprises a first current sensor 3310, a band-rejection filter 3311 anda harmonic calculating and amplifying circuit 3312. The first currentsensor 3310 detects the source current, and its output signal istransmitted to the band-rejection filter 3311 to filter out thefundamental component of the source current. The harmonic calculatingand amplifying circuit 3312 receives the output of the band-rejectionfilter 3311 and calculates the harmonic components of each specifiedorders and then amplifies them by different gains in accordance with thepreset attenuation ratio. It should be noticed that the band-rejectionfilter 3311 must be realized by means of analog circuit, and theharmonic calculating and amplifying circuit 3312 can be realized by amicroprocessor. Since the fundamental component is the dominatedcomponent of the source current after compensating by the active powerfilter, the harmonic components are too small to be detected from thesource current. Especially for the harmonic calculating and amplifyingcircuit 3312 being implemented by a microprocessor with limited bits ofA/D converters, an evident error will appear in calculating thecompensating components. This is the reason why such a band-rejectionfilter 3311 in the harmonic current loop 331 to filter out thefundamental component of the source current for emphasizing the harmoniccomponent must be realized by an analog circuit. Hence, the accuracy ofharmonic components calculation is improved.

One method to detect the source current is directly sensing it by usinga current sensor at the power source 1. There is the other method todetect the source current by using the current sensor at the load 2 todetect a current of the load 2 and calculating the source current byusing the current of the load 2 and the output current of the activepower filter 3.

Referring to 5, an equivalent circuit of the system shown in FIG. 3under the n^(th) harmonic frequency is illustrated. The active powerfilter 3 can be regarded as a dependent current source due to the use ofthe current-mode control of the inverter 31. The n^(th)-order harmoniccurrent of the load 2 is labeled as I_(Ln); the n^(th)-order harmoniccurrent of the power source 1 is labeled as I_(sn); and the inverter 31generates a n^(th)-order harmonic current KI_(sn) injecting into thepower line set 4, K represented as an amplification gain of the harmoniccalculating and amplifying circuit 3312. By using the Kirchhoff's rule,an equation as the following one is obtained.

$\begin{matrix}{I_{sn} = {\frac{1}{K + 1}I_{Ln}}} & (1)\end{matrix}$

According to the above equation (1), the amplification gain K determinesthe attenuating ratio of the n^(th)-order harmonic current. Therefore,while being applied to the harmonic current in specified order, theharmonic calculating and amplifying circuit 3312 not only obtains theharmonic components of each specified order but also determines theattenuating ratio thereof. Consequently, the output of the harmoniccalculating and amplifying circuit 3312 is the harmonic component of thereference signal I_(r)*.

Still referring to FIG. 4, the fundamental reactive current loop 332 ofthe controller 33 for the active power filter 3 is used to obtain afundamental reactive component of the reference signal I_(r)*. Due tothe current-mode control applied to the inverter 31, a current of acapacitor is shown as the follows:

$\begin{matrix}{i_{c} = {C\frac{V}{t}}} & (2)\end{matrix}$

where V is the voltage of power source 1. By using the fundamentalreactive current loop 332 to control the inverter 31, the compensatingcurrent generated by the active power filter 3 comprises a componentbeing proportional to a differential value of the fundamental voltage ofthe power source 1. As a result, the active power filter 3 acts as avirtual capacitor C at the fundamental frequency and paralleled connectsto the power source 1 for generating a reactive power to compensate theload 2. Please note that the active power filter 3 acts as a virtualcapacitor only at the fundamental frequency to avoid to resulting in aharmonic pollution.

Furthermore, the fundamental reactive current loop 332 comprises afundamental reactive power calculating circuit 3320, a first voltagesensor 3321, a band-pass filter 3322, a differentiating circuit 3323 anda first multiplier 3324. In order to determine a value of the virtualcapacitor for the required compensating reactive power of the activepower filter 3, the first voltage sensor 3321 senses the source voltage.The sensed source voltage and the output of the first current sensor3310 are transmitted to the fundamental reactive power calculatingcircuit 3320 to calculate the fundamental reactive power. Meanwhile, thesignal sensed by the first voltage sensor 3321 is also transmitted tothe band-pass filter 3322 to obtain a fundamental voltage of the powersource 1. Continuously, the output of the band-pass filter 3322 istransmitted to the differentiating circuit 3323 to generate adifferential signal of the fundamental voltage of the power source 1.Then, the outputs of the fundamental reactive power calculating circuit3320 and the differentiating circuit 3323 are transmitted to the firstmultiplier 3324 to generate the fundamental reactive component of thereference signal I_(r)*.

Thirdly, the fundamental real current loop 333 is employed for obtaininga fundamental real component of the reference signal I_(r)*. Due to thecurrent-mode control applied to the inverter 31, the current of thevirtual resistor is shown as the follows:

$\begin{matrix}{i_{R} = \frac{V}{R}} & (3)\end{matrix}$

Through the inverter 31, the fundamental real current loop 333 enablesthe active power filter 3 to generate a compensating current comprisinga component being proportional to the fundamental voltage of the powersource 1 shown as Eq. (3). Therefore, the active power filter 3 acts asa virtual resistor R at the fundamental frequency and connects to thepower source 1 in parallel to absorb a real power, so as to compensatefor the power loss of the active power filter 3 and maintain theregulated DC voltage level supplied by the energy storage capacitor 30.It should be noticed that the active power filter 3 only acts as avirtual resistor at the fundamental frequency to avoid to resulting in aharmonic pollution.

Furthermore, the fundamental real current loop 333 comprises a secondvoltage sensor 3330, a first subtracter 3331, a first controller 3332and a second multiplier 3333. The second voltage sensor 3330 detects thevoltage of the energy storage capacitor 30 and sends a signal to thefirst subtracter 3331 to compare with a predetermined signal. Thepredetermined signal is used to determine the desired voltage of energystorage capacitor 30. The output of the first subtracter 3331 istransmitted to the first controller 3332 for determining a value of thevirtual resistor at the fundamental frequency. Then, the outputs of thefirst controller 3332 and the band-pass filter 3322 are transmitted tothe second multiplier 3333 to generate the fundamental real component ofthe reference signal I_(r)*.

Finally, the fundamental reactive component generating by thefundamental reactive current loop 332, the fundamental real componentgenerated by the fundamental real current loop 333 and the harmoniccomponent generated by the harmonic current loop 331 are transmitted toa selecting circuit 334. The selecting circuit 334 selects the referencesignal I_(r)* from the combination of the above three components. Thecombination is shown as follows:

-   -   1. the reference signal I_(r)* is consisted of the fundamental        real component and the fundamental reactive component together        to compensate for the fundamental reactive power of the load 2;    -   2. the reference signal I_(r)* is consisted of the real        fundamental component and the harmonic component together to        suppress the harmonic currents of specific orders of the load 2;        and    -   3. the reference signal I_(r)* is consisted of all the harmonic        component, the fundamental reactive component and the        fundamental real component, in order to compensate for the        fundamental reactive power and suppress the harmonic currents of        specific orders of the load 2 simultaneously.

Still referring to FIG. 4, a second current sensor 335 detects theoutput current of the active power filter 3, and the output of thesecond current sensor 335 is send to a second subtracter 336 to comparewith the reference signal I_(r)*. The output of the second subtracter336 is transmitted to a second controller 337. The output of the secondcontroller 337 is send to a pulse-width modulation (PWM) circuit 338, soas to generate control signals for the power electronic switches of theinverter 31. With the close loop control system proposed by thepresented invention, the output current of the active power filter 3 cantrack the reference signal I_(r)*. The compensating current generated bythe active power filter 3 is injected into the power line set 4.

The described compensating current is the fundamental real componentoutputted by the fundamental real current loop 333 adding a selectablecompensation function from the combination of the fundamental reactivecomponent outputted by the fundamental reactive current loop 332 and/orthe harmonic component outputted by the harmonic current loop 331. Theinverter has the compensating current including the fundamental realcurrent component makes the active power filter 3 equal to a circuithaving a virtual resistor; and the inverter has the compensating currentincluding the fundamental reactive current component makes the activepower filter 3 equal to a circuit having a virtual capacitor. Theinverter has the compensating current including the harmonic currentcomponent makes the active power filter 3 to attenuate a compound of theharmonic currents of specific orders with a predetermined attenuatingratio set by user.

Referring to FIG. 6, a schematic circuitry of the active power filter 3in accordance with a second embodiment of the presented invention isillustrated. In comparison with the first embodiment, the inverter 31 ofthe second embodiment is configured by a four arms bridge structure,each arm is consisted of two series connected power electronic switches,and the neutral line 43 connects to a middle connected point of twoseries connected power electronic switches in one of the four arms.Additionally, the energy storage capacitor 30 has only one DC capacitor.

Referring to FIG. 7, a schematic circuitry of the active power filter 3in accordance with a third embodiment of the presented invention isillustrated. In comparison with the second embodiment, the active powerfilter 3 is applied to a three-phase three-wire power system, with thepower source 1 being a three-phase three-wire AC power source andsupplying the load 2 with a three-phase electrical power through thepower line set 4. In comparison with the second embodiment, the inverter31 of the third embodiment is configured by a three arms bridgestructure, and each arm is consisted of two series connected powerelectronic switches.

Referring to FIG. 8, a schematic circuitry of the active power filter 3in accordance with a fourth embodiment of the presented invention isillustrated. In comparison with the first through third embodiments, theactive power filter 3 is applied to a three-phase four-wire power systemof the power source 1 being a high level AC power source. Particularly,being different from the active power filter 3 disclosed by the firstembodiment, an AC power capacitor 34 is employed to serially connectbetween the filtering circuit 32 and the three power lines 40, 41, 42.This AC power capacitor 34 supplies an additional reactive power andblocks most of the fundamental voltage of the power source 1 while theactive power filter 3 is applied in the power system with higher voltagelevel. Additionally, the fundamental reactive current loop 332 of thecontroller 33 only requires to set a low limited value, and it canreduce the capacity of the inverter 31 used in the active power filter3.

Referring to FIG. 9, a schematic circuitry of the active power filter 3in accordance with a fifth embodiment of the presented invention isillustrated. In comparison with the fourth embodiment, the inverter 31of the fifth embodiment can also be configured by a four arms bridgestructure, each arm is consisted of two series connected powerelectronic switches, and the neutral line 43 connects to a middleconnected point of two series connected power electronic switches in oneof the four arms. Additionally, the energy storage capacitor 30 has onlyone DC capacitor.

Referring to FIG. 10, a schematic circuitry of the active power filter 3in accordance with a sixth embodiment of the presented invention isillustrated. In comparison with the fifth embodiment, the active powerfilter 3 is applied to a three-phase three-wire power system of thepower source 1 being a high level AC power source. Furthermore, beingdifferent from the active power filter 3 disclosed by the fifthembodiment, the inverter 31 of the fifth embodiment is configured by athree arms bridge structure, and each arm is consisted of two seriesconnected power electronic switches.

Although the invention has been described in detail with reference toits presentedly preferred embodiment, it will be understood by one ofordinary skill in the art that various modifications can be made withoutdeparting from the spirit and the scope of the invention, as set forthin the appended claims.

1. An active power filter, comprising: an energy storage capacitorproviding a regulated DC voltage; an inverter including a plurality ofpower electronic switches, with the inverter electrically connectingwith two terminals of the energy storage capacitor for switching theregulated DC voltage; a filtering circuit providing two sets ofterminals with one of the sets electrically connecting to the inverterand the other electrically connecting to power lines between a powersource and a load; and a controller outputting control signalstransmitted to the inverter for driving the power electronic switches;wherein the controller detects a source current, a source voltage and anenergy storage capacitor voltage for calculating a reference signal anddetects an output current of the active power filter, the output currenttracking the reference signal through a feedback control of thecontroller, so as to control the power electronic switches of theinverter generating a compensating current.
 2. The active power filteras defined in claim 1, wherein the compensating current comprises acomponent being proportional to a fundamental voltage of the powersource for controlling the active power filter acting as a virtualresistor at fundamental frequency and connecting with the power sourcein parallel to compensate for the power loss of the active power filter.3. The active power filter as defined in claim 1, wherein thecompensating current comprises a component being proportional to adifferential value of a fundamental voltage of the power source forcontrolling the active power filter acting as a virtual capacitor atfundamental frequency and connecting with the power source in parallelto compensate for the fundament reactive power of the load and the powerloss of the active power filter
 3. 4. The active power filter as definedin claim 1, wherein the compensating current comprises harmoniccomponents of specific orders for operating the active power filter toattenuate a compound of the harmonic currents of specific orders with apredetermined attenuating ratio.
 5. The active power filter as definedin claim 1, wherein the controller further comprises a harmonic currentloop, a fundamental reactive current loop and a fundamental real currentloop.
 6. The active power filter as defined in claim 5, wherein theharmonic current loop comprises a first current sensor, a band-rejectionfilter and a harmonic calculating and amplifying circuit, with the firstcurrent sensor detecting the source current and transmitting a signalthrough the band-rejection filter to filter out the signal of afundamental component, the output of the band-rejection filter transmitsto the harmonic calculating and amplifying circuit to calculate theharmonic component of each specified order and then amplifies them bydifferent gains in accordance with the predetermined attenuationthereof.
 7. The active power filter as defined in claim 6 furthercomprises a current sensor for detecting a current of the load andcalculating the source current by the current of the load and the outputcurrent of the active power filter.
 8. The active power filter asdefined in claim 5, wherein the reactive power circuit comprises afundamental reactive power calculating circuit, a first voltage sensor,a band-pass filter, a differentiating circuit and a first multiplier,with the first voltage sensor sensing the source voltage, and the outputsignals of the first voltage sensor and a first current sensor aretransmitted to the fundamental reactive power calculating circuit forcalculating a value of the virtual capacitor of the inverter, with thesignal generated by the first voltage sensor being transmitted to theband-pass filter to obtain a fundamental voltage of the power source,with the output signals of the fundamental reactive power calculatingcircuit and the differentiating circuit are transmitted to the firstmultiplier to generate a signal being proportional to a differentialvalue of the fundamental voltage of the power source.
 9. The activepower filter as defined in claim 5, wherein the real power circuitcomprises a second voltage sensor, a first subtracter, a firstcontroller and a second multiplier, with the second voltage sensordetecting the energy storage capacitor voltage and sending a signal tothe first subtracter together with a predetermined signal, with theoutput of the first subtracter being transmitted to the first controllerfor setting a value of the virtual resistor of the inverter, with theoutputs of the first controller and a band-pass filter are transmittedto the second multiplier to obtain a fundamental voltage of the powersource to generate a signal being proportional to the fundamentalvoltage of the power source.
 10. The active power filter as defined inclaim 5, wherein the controller further comprises a selecting circuitselecting the reference signal from a combination from the outputs ofthe harmonic current loop, the fundamental reactive current loop and thefundamental real current loop.
 11. The active power filter as defined inclaim 10, wherein the selecting circuit selects a combination by addingthe outputs of the fundamental reactive current loop and the fundamentalreal current loop together to compensate for the fundamental reactivepower of the load.
 12. The active power filter as defined in claim 10,wherein the selecting circuit selects a combination by adding theoutputs of the fundamental real current loop and the harmonic currentloop together to suppress the harmonic currents of specific orders ofthe load.
 13. The active power filter as defined in claim 10, whereinthe selecting circuit selects a combination by adding the outputs of theharmonic current loop, the fundamental reactive current loop and thefundamental real current loop together to compensate the fundamentalreactive power and suppress the harmonic currents of specific orders ofthe load.
 14. The active power filter as defined in claim 5, wherein thecontroller further comprises a second current sensor, a secondsubtracter, a second controller and a pulse width modulation (PWM)circuit, with the second current sensor detecting an output current ofthe active power filter, the outputs of the second current sensor andthe reference signal are transmitted to the second subtracter, and theoutput of the second subtracter are transmitted to the secondcontroller, then the output of the second controller are sent to the PWMcircuit, so as to generate control signals for the power electronicswitches within the inverter.
 15. The active power filter as defined inclaim 1, wherein applying the active power filter to a three-phasefour-wire power system the inverter is configured by a three arms bridgestructure, each arm is consisted of two series connected two powerelectronic switches, with the energy storage capacitor being formed bytwo serially connecting DC capacitors.
 16. The active power filter asdefined in claim 1, wherein applying the active power filter to athree-phase four-wire power system the inverter is configured by a fourarms bridge structure, and each arm is consisted of two series connectedpower electronic switches, with the energy storage capacitor beingformed by single DC capacitor.
 17. The active power filter as defined inclaim 1, wherein applying the active power filter to a three-phasethree-wire power system the inverter is configured by a three armsbridge structure, and each arm is consisted of two series connectedpower electronic switches, with the energy storage capacitor beingformed by single DC capacitor.
 18. The active power filter as defined inclaim 1, further comprising an AC power capacitor employed to connectwith the filtering circuit in series and link with the power lines forapplying to a power source with higher voltage level.